1. Field of the Invention
The present invention relates to strain sensors and, more particularly, strain sensors embedded in an elastomeric material for measuring forces along three axes.
2. Description of the Related Art
Conventional strain gauges or sensors are typically used for measuring the expansion and/or contraction of an object under stress. Strain sensors may comprise a resistive transducer, the electric output of which is proportional to the amount it is deformed under strain. In one type of resistive strain gauge illustrated in FIG. 1, the gauge 1 is made of a metal foil or metal wire 2 that is mounted on a substrate 3, wherein the wire changes resistance with expansion or contraction in a particular direction. FIG. 1 illustrates movement of the gauge, which is indicative of movement of the object being monitored, with the arrow “x” indicating movement in the “x” direction. Such a sensor requires either a DC or an AC excitation voltage to generate a strain signal. In addition, it is preferably connected in a differential arrangement such as in a Wheatstone bridge circuit to determine the amount of strain. Other types of strain sensors include parallel plate capacitors, piezoresistive silicon strain gauges, piezoelectric devices such as lead zirconium titanate (PZT), capacitors formed of inter-digitized fingers simulating adjacent parallel-plate capacitors, conductive elastomer resistive strain gauges, etc.
Each of these strain sensors is adapted to measure strain forces exerted on an object in a particular direction. However, measuring strain in three axes is often desirable. For example, multiple axis strain detection is often of particular concern in determining shear and compressive strain in an elastomeric tire. Monitoring the forces exerted on the tread rubber of a tire in multiple axes can provide an indication as to the performance of the tire (e.g., traction), as well as provide information valuable, for example, in controlling different components of a vehicle. According to one type of tire monitoring sensor, the deflection of tire tread is measured as it passes through a contact patch, the contact patch being defined by that portion of the tire in contact with the road at any particular time. The sensor in this device is a piezoelectric polymer, a photo restrictive fiber optic, a variable plate capacitor, or a variable inductor, each of which is capable of measuring the length of the contact patch during tire operation. In addition, the sensor is connected to a transponder device for communicating single-axis strain data for analysis. Most notably, the data obtained by such a sensor does not provide any useful traction information because it is only capable of measuring the length of the contact patch. As a result, variables which affect the coefficient of friction, such as road condition, are ignored. Overall, this sensor is unable to provide sufficient data for determining tri-axial strain forces of interest.
According to another known type of tire sensing device, a number of toroidal bands of piezoresistive or piezoelectric material are disposed in the tread of the tire. Notably, the measurement obtained by this device is not localized to a single tread block, and as a result, suffers from undesirable effects due to centrifugal force, road surface irregularities, and pressure changes. In yet another sensor device for monitoring tires, reed sensors incorporating strain gauges are employed, each sensor measuring forces directed in a single axis. In this arrangement, three separate devices, disposed at three separate locations, are required to obtain three axes of traction data. A significant problem associated with such a device is that each individual tread block will experience forces from the three axes concurrently. Typically, each tread block acts independently in a stick-slip fashion. As a result, measuring X axis data from one tread block, Y axis data from an adjacent tread block and Z axis data from yet another location, will yield three axes of data that is of little use.
In view of the above, the field of sensor devices was in need of a sensor assembly that measures strain in three dimensions at a particular point or region so as to monitor, for example, tire traction, etc. Moreover, such a device should be self-contained contained and be capable of being embedded in an object to be monitored, such as an elastomeric material (e.g., the rubber of a tire), during manufacture of the object without compromising the integrity of its performance.